[IMP] point_of_sale: make certain widgets extendable

This makes patching much easier. In some cases we can work around some
of these widgets not being exported, but it's ugly and there's just no
real reason to do that.

addons/point_of_sale/static/src/js/screens.js

@@ -1915,6 +1915,8 @@ return {
     ScreenWidget: ScreenWidget,
     PaymentScreenWidget: PaymentScreenWidget,
     OrderWidget: OrderWidget,
+    NumpadWidget: NumpadWidget,
+    ProductScreenWidget: ProductScreenWidget,
 };
 
 });